The present invention generally relates to a field emission display (FED) device and a method for fabricating the device and more particularly, relates to a field emission display device with a diode structure and equipped with nanotube emitters as the electron emission source and a method for fabricating the device by a thick film printing technique.
In recent years, flat panel display devices have been developed and widely used in electronic applications such as personal computers. One of the popularly used flat panel display device is an active matrix liquid crystal display which provides improved resolution. However, the liquid crystal display device has many inherent limitations that render it unsuitable for a number of applications. For instance, liquid crystal displays have numerous fabrication limitations including a slow deposition process for coating a glass panel with amorphous silicon, high manufacturing complexity and low yield for the fabrication process. Moreover, the liquid crystal display devices require a fluorescent backlight which draws high power while most of the light generated is wasted. A liquid crystal display image is also difficult to see under bright light conditions or at wide viewing angles which further limit its use in many applications.
Other flat panel display devices have been developed in recent years to replace the liquid crystal display panels. One of such devices is a field emission display device that overcomes some of the limitations of LCD and provides significant advantages over the traditional LCD devices. For instance, the field emission display devices have higher contrast ratio, larger viewing angle, higher maximum brightness, lower power consumption and a wider operating temperature range when compared to a conventional thin film transistor (TFT) liquid crystal display panel.
A most drastic difference between a FED and a LCD is that, unlike the LCD, FED produces its own light source utilizing colored phosphors. The FEDs do not require complicated, power-consuming backlights and filters and as a result, almost all the light generated by a FED is visible to the user. Furthermore, the FEDs do not require large arrays of thin film transistors, and thus, a major source of high cost and yield problems for active matrix LCDs is eliminated.
In a FED, electrons are emitted from a cathode and impinge on phosphors coated on the back of a transparent cover plate to produce an image. Such a cathodoluminescent process is known as one of the most efficient methods for generating light. Contrary to a conventional CRT device, each pixel or emission unit in a FED has its own electron source, i.e., typically an array of emitting microtips. A voltage difference existed between a cathode and a gate electrode which extracts electrons from the cathode and accelerates them toward the phosphor coating. The emission current, and thus the display brightness, is strongly dependent on the work function of the emitting material. To achieve the necessary efficiency of a FED, the cleanliness and uniformity of the emitter source material are very important.
In order for the electron to travel in a FED, most FEDs are evacuated to a low pressure such as 10xe2x88x927 torr in order to provide a log mean free path for the emitted electrons and to prevent contamination and deterioration of the microtips. The resolution of the display can be improved by using a focus grid to collimate electrons drawn from the microtips.
In the early development for field emission cathodes, a metal microtip emitter of molybdenum was utilized. In such a device, a silicon wafer is first oxidized to produce a thick silicon oxide layer and then a metallic gate layer is deposited on top of the oxide. The metallic gate layer is then patterned to form gate openings, while subsequent etching of the silicon oxide underneath the openings undercuts the gate and creates a well. A sacrificial material layer such as nickel is deposited to prevent deposition of nickel into the emitter well. Molybdenum is then deposited at normal incidence such that a cone with a sharp point grows inside the cavity until the opening closes thereabove. An emitter cone is left when the sacrificial layer of nickel is removed.
In an alternate design, silicon microtip emitters are produced by first conducting a thermal oxidation on silicon and then followed by patterning the oxide and selectively etching to form silicon tips. Further oxidation or etching protects the silicon and sharpens the point to provide a sacrificial layer. In another alternate design, the microtips are built onto a substrate of a desirable material such as glass, as an ideal substrate for large area flat panel displays. The microtips can be formed of conducting materials such as metals or doped semi-conducting materials. In this alternate design for a FED device, an interlayer that has controlled conductivity deposited between the cathode and the microtips is highly desirable. A proper resistivity of the interlayer enables the device to operate in a stable condition. In fabricating such FED devices, it is therefore desirable to deposit an amorphous silicon film which has electrical conductivity in an intermediate range between that of intrinsic amorphous silicon and n+ doped amorphous silicon. The conductivity of the n+ doped amorphous silicon can be controlled by adjusting the amount of phosphorous atoms contained in the film.
Generally, in the fabrication of a FED device, the device is contained in a cavity of very low pressure such that the emission of electrons is not impeded. For instance, a low pressure of 10xe2x88x927 torr is normally required. In order to prevent the collapse of two relatively large glass panels which form the FED device, spacers must be used to support and provide proper spacing between the two panels. For instance, in conventional FED devices, glass spheres or glass crosses have been used for maintaining such spacings in FED devices. Elongated spacers have also been used for such purpose.
Referring initially to FIG. 1A wherein an enlarged, cross-sectional view of a conventional field emission display device 10 is shown. The FED device 10 is formed by depositing a resistive layer 12 of typically an amorphous silicon base film on a glass substrate 14. An insulating layer 16 of a dielectric material and a metallic gate layer 18 are then deposited and formed together to provide metallic microtips 20 and a cathode structure 22 is covered by the resistive layer 12 and thus, a resistive but somewhat conductive amorphous silicon layer 12 underlies a highly insulating layer 16 which is formed of a dielectric material such as SiO2. It is important to be able to control the resistivity of the amorphous silicon layer 12 such that it is not overly resistive but yet, it will act as a limiting resistor to prevent excessive current flow if one of the microtips 20 shorts to the metal layer 18.
A completed FED structure 30 including anode 28 mounted on top of the structure 30 is shown in FIG. 1B. It is to be noted, for simplicity reasons, the cathode layer 22 and the resistive layer 12 are shown as a single layer 22 for the cathode. The microtips 20 are formed to emit electrons 26 from the tips of the microtips 20. The gate electrodes 18 are provided with a positive charge, while the anode 28 is provided with a higher positive charge. The anode 28 is formed by a glass plate 36 which is coated with phosphorous particles 32. An intermittent conductive layer of indium-tin-oxide (ITO) layer 34 may also be utilized to further improve the brightness of the phosphorous layer when bombarded by electrons 26. This is shown in a partial, enlarged cross-sectional view of FIG. 1C. The total thickness of the FED device is only about 2 mm, with vacuum pulled inbetween the lower glass plate 14 and the upper glass plate 36 sealed by sidewall panels 38 (shown in FIG. 1B).
The conventional FED devices formed by microtips shown in FIGS. 1Axcx9c1C produce a flat panel display device of improved quality when compared to liquid crystal display devices. However, a major disadvantage of the microtip FED device is the complicated processing steps that must be used to fabricate the device. For instance, the formation of the various layers in the device, and specifically the formation of the microtips, requires a thin film deposition technique utilizing a photolithographic method. As a result, numerous photomasking steps must be performed in order to define and fabricate the various structural features in the FED. The CVD deposition processes and the photolithographic processes involved greatly increase the manufacturing cost of a FED device.
In a copending application, Attorney""s Docket No. 64,600-050, assigned to the common assignee of the present invention, a field emission display device and a method for fabricating such device of a triode structure using nanotube emitters as the electron emission sources were disclosed. In the triode structure FED device, the device is constructed by a first electrically insulating plate, a cathode formed on the first electrically insulating plate by a material that includes metal, a layer formed on the cathode of a high electrical resistivity material, a layer of nanotube emitter formed on the resistivity layer of a material of carbon, diamond or diamond-like carbon wherein the cathode, the resistivity layer and the nanotube emitter layer form an emitter stack insulated by an insulating rib section from adjacent emitter stacks, a dielectric material layer perpendicularly overlying a multiplicity of the emitter stacks, a gate electrode on top of the dielectric material layer, and an anode formed on a second electrically insulating plate overlying the gate electrode. The FED device proposed can be fabricated advantageously by a thick film printing technique at substantially lower fabrication cost and higher fabrication efficiency than the FEDs utilizing microtips. However, three separate electrodes are still required for the device, i.e., a cathode, a gate electrode and an anode. It is therefore desirable to design a new FED device that has a simplified structure without the need for three separate electrodes in order to function.
It is therefore an object of the present invention to provide a FED device that can be fabricated by a thick film printing technique that does not have the drawbacks or shortcomings of a conventional FED device.
It is another object of the present invention to provide a FED device that can be fabricated by a thick film printing technique in a simplified structure when compared to a triode structured FED device.
It is a further object of the present invention to provide a FED device that can be fabricated by a thick film printing technique without the need for thin film deposition and photolithographic processes.
It is another further object of the present invention to provide a FED device that can be fabricated by a screen printing technique at a substantially lower fabrication cost.
It is still another object of the present invention to provide a FED device that utilizes a diode structure of a single cathode and a single anode.
It is yet another object of the present invention to provide a FED device that can be fabricated by a thick film printing technique for forming nanotube emitter layers from nanometer dimensioned hollow fibers made of a carbon, a diamond or a diamond-like carbon material.
It is still another further object of the present invention to provide a method for fabricating a FED device by a thick film printing technique in a diode structure in which a multiplicity of spaced-apart emitter stacks are formed by screen printing a nanotube emitter material on an electrically conductive silver paste layer.
It is yet another further object of the present invention to provide a method for fabricating a FED device utilizing a thick film printing technique to form a cathode layer, a nanotube emitter layer, insulating rib sections on a bottom insulating plate and a fluorescent powder coating layer on a top insulating plate.
In accordance with the present invention, a field emission display device that has a diode structure and a method for fabricating such device are disclosed.
In a preferred embodiment, a field emission display panel is provided which includes a first electrically insulating plate, a plurality of emitter stacks formed on the first electrically insulating plate, each of the emitter stacks is positioned parallel to a transverse direction of the first insulating plate and includes a layer of a first electrically conductive material and a layer of nanotube emitter on top, a plurality of rib sections formed of an insulating material inbetween the plurality of emitter stacks providing electrical insulation thereinbetween, a second electrically insulating plate positioned over and spaced-apart from the first electrically insulating plate that has an inside surface facing the first plate, a layer of a second electrically conductive material on the inside surface of the second insulating plate, a multiplicity of strips of flourescent powder coating on the second electrically conductive material each for emitting a red, green or blue light upon activation by electrons emitted from the plurality of emitter stacks, and a plurality of side panels joining peripheries of the first and the second electrically insulating plates together forming a vacuum-tight cavity therein.
In the field emission display panel, the second electrically insulating plate further includes a black matrix layer inbetween the multiplicity of strips of fluorescent powder coating. The black matrix layer may be formed of an electrically conductive material. The first and the second electrically insulating plates are formed of a ceramic material that is substantially transparent. The layer of the first electrically conductive material may be a cathode for the field emission display panel, the layer of the first electrically conductive material may be a silver paste.
In the field emission display panel, the layer of the second electrically conductive material may be an anode for the field emission display panel, and may be formed of indium-tin-oxide (ITO). The layer of nanotube emitter may be formed of a mixture of nanometer dimensioned hollow tubes (or fibers) and a binder material, or a mixture of nanotube dimensioned hollow fibers of carbon, diamond or diamond-like carbon and a polymeric based binder. Each of the multiplicity of strips of fluorescent powder coating emits a light of red, green or blue that is different than the light emitted by its immediate adjacent strips when activated by electrons from the plurality of emitter stacks. Each of the plurality of rib sections further includes a base portion that overlaps an end portion of an emitter stack for reducing edge emission and a top portion extending from the base portion for confining electrons emitted from the emitter stack.
The present invention is further directed to a method for fabricating a field emission display panel that has a diode structure by the operating steps of providing a first electrically insulating plate, forming a plurality of emitter stacks on the first electrically insulating plate by a thick film printing technique parallel to a transverse direction of the first plate, each of the emitter stacks includes a layer of a first electrically conductive material and a layer of nanotube emitter on top, forming a plurality of rib sections from an electrically insulating material inbetween the plurality of emitter stacks providing electrical insulation thereinbetween, providing a second electrically insulating plate, forming a layer of a second electrically conductive material on an inside surface of the second electrically insulating plate facing the first electrically insulating plate when the first and the second plates are assembled together, forming a multiplicity of strips of fluorescent powder coating on the layer of second electrically conductive material for emitting a red, green or blue light when activated by electrons, and joining the first and the second electrically insulating plates together by side panels forming a vacuum-tight cavity therein.
The method for fabricating a field emission display panel that has a diode structure may further include the step of providing the first and the second electrically insulating plates in substantially transparent glass plates. The method may further include the step of printing the layer of a first electrically conductive material in a silver paste. The method may further include the step of printing the layer of nanotube emitter from a mixture of a binder and nanotube dimensioned hollow fibers of carbon fibers, diamond fibers or diamond-like carbon fibers. The method may further include the step of connecting a negative charge to the first electrically conductive material under the plurality of emitter stacks and a positive charge to the layer of the second electrically conductive material. The layer of the second electrically conductive material may be indium-tin-oxide (ITO).
The method for fabricating a field emission display panel that has a diode structure may further include the step of coating a black matrix layer on the second electrically insulating plate inbetween the multiplicity of strips of fluorescent powder coating. The multiplicity of strips of fluorescent powder coating may be formed by a thick film printing technique. The multiplicity of strips of fluorescent powder coating may be formed such that each strip emits a red, green or blue light that is different than its immediate adjacent strips when activated by electrons from the plurality of emitter stacks. The method may further include the step of forming the plurality of rib sections such that each rib section overlaps an end portion of an emitter stack for reducing edge emission from the emitter stack. The method may further include the step of forming the plurality of rib sections such that a top portion of each rib section extends upwardly from a base portion for confining electrons emitted from the emitter stack. The method may further include the step of forming the multiplicity of strips of fluorescent powder coating by a material including phosphor.